Begin by first logging into your CircleCI account to access the dashboard.
Next click the settings button in the left navigation panel.
From here select "Plan Settings". In the new panel on the right, you can view how many containers are currently in your plan. From here you can modify the number of containers and update your plan.
To cancel your plan entirely and drop to the free package, change your paid containers to 0 and the "Pay Now" button will change to "Cancel Plan".